I don't know whether PHP does anything special in this case, but for the record, \ and / have always been interchangeable in pathnames passed to any DOS or Windows API. Even in command lines, / and \ are still equivalent provided the shell can be persuaded not to parse / as a switch prefix.

So if you're building stuff that needs to be portable, and it has relative pathnames embedded in it, just use / for all pathname separators. As a bonus, it makes your pathnames look less retarded in languages that use \ for an escape prefix in string literals.